Frangible molds



July. 14, 1970 D. F. CAREY FRANGIBLE MOLDS Filed July 17, 1967 FIG. 4

INVENTOR. DONALD E CAREY AGENT United States Patent 3,520,509 FRANGIBLE MOLDS Donald F. Carey, Brigham City, Utah, assignor to Thiokol Chemical Corporation, Bristol, Pa., a corporation of Delaware Filed July 17, 1967, Ser. No. 653,691 Int. Cl. B28b 7/34 U.S. Cl. 249-61 2 Claims ABSTRACT OF THE DISCLOSURE A frangible mold for forming radial slots in a hollow body is made by driving nails through a rigid sheet, winding a tape about the nails, molding a plastic on the sheet so that the tape is embedded therein. and removing the sheet and nails.

This invention relates to molding pieces for forming radial slots in hollow bodies; and, more particularly, it relates to frangible'molds that can be readily removed from the casting.

At least one radial slot adjoining a central, circular, cylindrical or conical cavity has become a much used burning-surface configuration in solid-propellant grains for rockets. The central cavity is usually formed simply by casting a mandrel into the propellant and withdrawing it after the propellant has cured. The radial slot, however, has been more difiicult to form conveniently, inexpensively, and with precision.

The present invention is a frangible mold for forming such radial slots. This mold is a slab of plastic foam with a continuous tape embedded therein in such a manner that pulling on a free end of the tape will fragment the mold for removal thereof from the cured propellant. The invention also includes a method of making the mold.

An object of the invention is to provide a convenient and inexpensive means of making radial slots in a hollow body, such as a rocket grain.

Another object of the invention is to provide a mold for making such slots that may be easily removed from the hollow body after casting and curing thereof.

Another object of the invention is to provide a convenient means of making such a mold.

Other objects and advantages of the invention will become apparent as it is disclosed in the following description and accompanying drawings, wherein identical characters designate the same parts throughout the disclosure.

In the drawings:

FIG. 1 is a perspective view of one component of a preferred embodiment of the invention in an early stage of manufacture;

FIG. 2 is a view similar to FIG. 1 at a later stage of manufacture;

FIG. 3 is a perspective view of plurality of completed components assembled to form a mold; and

FIG. 4 is a fragmentary section of a rocket motor showing a radial slot formed by the mold of FIG. 3.

FIG. 1 illustrates a rigid sheet having the shape of a radial segment 11 of the mold 12. Nails 14 have been driven through the underside 16 of the sheet 10, the points 18 projecting upwardly beyond the upper surface 20 thereof. A continuous tape 22, of a strong, fairly stiff material such as paper reinforced with glass fibers, is wound about the nails 14 in a manner to divide the upper surface 20 of the sheet 10 into small areas (see FIG. 2). The nails 14, being perpendicular to the sheet 10 also automatically orient the tape 22 perpendicular to the parallel top and 3,5205%9 Patented July 14, 1970 bottom surfaces of the segment 11. A free end 26 of the tape 22 extends beyond the sheet 10 at the inner edge 28 thereof. A moldable, frangible material 30, such as a polyurethane foam of about two pounds per cubic foot density is then cast on the sheet 10, with the aid of a partial enclosure not shown by which the top and sides of the slab of foam are formed. The tape 22 is completely embedded in the foam 30, except for the free end 26. Various other kinds of material especially those polymeric in nature, may be used for the frangible material 30.

After the frangible material 30 has cured, it is removed from the sheet 10 and the nails 14. If desired, the holes left by the nails 14 in the mold segment 11 may be filled with nearly any moldable material to provide smooth top and bottom surfaces on the mold segments 11. A plurality of the resulting mold segments 11 are then assembled to form the annular mold 12.

The case 32 of a rocket motor 34 (see FIG. 4) is then positioned vertically and filled with uncured propellant 36 about a central mandrel, not shown, up to the level at which a radial slot 38 is to be made. This propellant 36 is cured and the mold 12 is arranged on the upper surface 40 thereof. The remainder 42 of the propellant is cast in the case 32. and cured; after which a tool may be inserted through the igniter port 44 of the case 32 to grasp each free end 26 of tape 22 and pull thereon to fragment the mold segments 11 for removal from the propellant gram.

An invention has been described for making radial slots in a molded, hollow body. Although a preferred form thereof has been described with considerable specificity with regard to detail, it is understood that such details may be altered without departing from the scope of the invention as defined in the appended claims. For example, the mold 12 is not necessarily planar in form, but may be conical, etc.

What is claimed is:

1. A mold comprising:

a slab of frangible, foamed polymeric material; and

a continuous tape embedded in said slab with a free end projecting therefrom, said tape being oriented perpendicular to parallel surfaces of said slab and arranged to divide said slab into small portions, whereby said slab may be fragmented into said small portions when said free end of said tape is pulled.

2. The process for making a frangible mold comprising the steps of:

driving nails through one side of a rigid sheet;

winding a continuous tape on said nails on one side of said sheet, said tape being perpendicular to said sheet and arranged to divide the surface of said sheet into small areas;

molding an easily-frangible material on said sheet so that said tape is embedded therein, with one end of said tape extending from said frangible material; allowing said frangible material to harden; and removing said sheet and said nails therefrom.

References Cited UNITED STATES PATENTS 2,343,292 3/1944 Greneker 24961 2,364,710 12/ 1944 Greneker 24961 3,136,002 6/1964 Schwartz 264-3 SAMUEL FEINBERG, Primary Examiner U.S. Cl. X.R. 60254; 264-3, 221 

